This application generally relates to the field of producing elongated strands from highly viscous liquid materials, and more particularly to the creation of optical fibers from molten glass using focused fluid technology.
Optical fibers are thin strands of materials, such as glass or polymeric compounds, capable of transmitting an optical signal containing a large amount of information over long distances with very low loss. (See U.S. Pat. Nos. 6,128,429; 6,098,428; 6,057,034 and publications and patents cited in each of these patents) Optical communication systems based on glass optical fibers allow communication signals to be transmitted over long distances with low attention and at extremely high data rates, or band width capacity. This capability arises form the propagation of a single optical signal mode in the low loss windows of glass located at the near-infrared wavelengths. Since the introduction of erbium doped fiber amplifier (EDFA), the last decade has witnessed the emergence of the glass optical fiber as the standard data transmission medium for wide area networks (WANs).
Conventional optical fibers are typically manufactured by constructing an optical fiber preform of appropriate composition and drawing a fiber from the preform. (See U.S. Pat. No. 6,053,012 and patents and publications cited therein) A typical preform is a concentric glass rod having a length of about one meter and an outer diameter of 20-200 mm. The inner core of the rod is a high purity, low loss glass such as germanium silicate glass having a diameter of about 1-5 mm. The concentric outer cylinder, referred to as cladding, is a layer of glass with a lower index of refraction than the inner core.
In the conventional manufacture of an optical fiber, the preform is lowered into the insulated susceptor of an RF induction furnace where it is heated to a high drawing temperature. (See U.S. Pat. Nos. 5,741,384; 5,698,124 and patents and publications cited in each) A strand of glass is pulled from the heated portion of the preform at a tension and rate to draw a fiber of desired diameter. One of the primary difficulties with this conventional process is contamination of the fiber from the materials of the induction furnace. Even very small particulates from the insulation or susceptor can produce localized weak points in the fiber which will ultimately result in breakage or other forms of failure. U.S. Pat. No. 4,440,556 describes an early attempt to solve this contamination problem by directing a plasma torch axially onto a preform and drawing a fiber axially through a central passage in the torch. The difficulty with this approach is that to reach the central passage, the drawn fiber must pass through the plasma fireball. But plasma shapes are notoriously difficult to control, and even minor fluctuations in shape can subject the delicate drawn fiber to severe temperature fluctuations.
Another difficulty arises from the use of increasingly larger diameter preforms. With larger diameter preforms it is very difficult to generate a sufficiently large plasma fireball to cover the entire diameter of the preform. The result is non-uniform heating in the drawing region. Similar methods, such as the method described in U.S. Pat. No. 5,672,192, address some of the problems inherent in these methods, but still requires the use of a plasma torch and thus has many of the limitations inherent to this use.
The success of the single-mode glass optical fiber in communication backbones has given rise to the concept of optical networking. These networks serve to integrate data streams over all optical systems as communication signals make their way from WANs down to smaller local area networks (LANs) and eventually to the end user by fiber to the desktop. The increased use of optical networks, based in large part on the recent explosion of the Internet and use of the World Wide Web, has demanded vastly higher bandwidth performance in short-and medium-based applications.
There is thus a need in the art for improved methods of producing glass optical fibers to meet the growing demands of consumer use. In addition, there is a growing demand for better optical fibers, both single mode and multimode optical fibers.
The invention is directed to the production of optical fibers using flow physics. The present methods provide for the focused extrusion of a highly viscous material such as molten germanium silicate glass, either directly from a molten liquid or from a perform, using a fluid (e.g. a heated gas or liquid) that surrounds and focuses the high viscosity liquid stream or preform, resulting in an evenly shaped, elongated fiber. The invention also provides methods and devices for the manufacture of optical performs, which can then be drawn using conventional technology or using the drawing methods disclosed herein.
A flow physics methodology which is applied to low viscosity fluids is described in publications such as U.S. Pat. Nos. 6,116,516 issued Sep. 12, 2000; 6,187,214 issued Sep. 13, 2001; 6,197,835 issued Mar. 6, 2001; and 6,196,525 issued Mar. 6, 2001. However, these disclosures relate to the extrusion of low viscosity fluids. What is mean by low viscosity fluid is that the fluid has a Reynolds number which is relatively high, for example a number about 10 or more. The extrusion of low viscosity fluids is carried out under conditions using forces which are not dominated by the viscosity of the fluid but rather dominated by the mass of the fluid or its density. By analogy, the engine of car moves the car forward using the power of the engine largely to have an effect on the mass of the car and, to a lesser extent, in order to overcome the frictional resistances existing between various components. However, if the frictional forces are substantially increased such as by applying the emergency brake of the car then there frictional forces become the dominant forces which must be overcome in order to move the car forward. In this analogy the frictional forces relate to the viscosity of the fluid.
The disclosure provided here is directed towards methodology which describes creating streams and fibers from high viscosity fluids. The term xe2x80x9chigh viscosity fluidxe2x80x9d is intended to encompass fluids wherein the Reynolds number is relatively small, specifically a Reynolds number of about 1 or less. More particularly, the Reynolds number in a very high viscosity fluid is less than about 0.1. With high viscosity fluids, as with the car with the emergency brake on, the viscosity of the fluid becomes a dominating factor in terms of what must be overcome by the forces applied in order to move the fluid forward just as the frictional resistance created by the emergency brake becomes the dominating factor which the car engine must overcome in order to move the car forward.
A section entitled xe2x80x9cmathematical formulationxe2x80x9d is included below. This section includes equations which will be understood by those skilled in the art upon reading this disclosure as applicable to the manufacturing of streams and fibers from high viscosity fluids such as the high viscosity fluid of molten silica glass with a high viscosity fluid of a heated glass preform used in creating fibers which are used to optically transmit information.
In a first embodiment, elongated fibers such as optical fibers are produced directly from a highly viscous liquid, e.g., molten silicate glass, by subjecting a stream of the viscous liquid to a surrounding, focusing fluid. This allows fibers to be generated without the need for producing a perform, and can also allow the extrusion of multiple fibers simultaneously. This extrusion is particularly advantageous in that the fiber stream does not contact the surface of the orifice upon extrusion of the fiber from the devices of the invention because the extruded fiber is completely surrounded by and focused with the focusing fluid which may be a gas. This makes it possible to reduce contamination of the fiber and essentially prevents clogging of the device orifice. Elongated fibers produced can have any desired diameter but are preferably 200 microns or less in diameter and may be from 1 micron to 50 microns in diameter.
In another embodiment, optical fiber preforms are reduced in diameter and increased in length using the focusing properties of a surrounding fluid. The optical fiber precursors (i.e. the preforms) are heated to a temperature that allows the preform material to maintain the basic structural elements of the preforms while allowing the preform to become ductile or specifically to be stretched to the desired length and lateral dimensions, i.e. a temperature which renders the optical fiber precursor ductile and allows the fiber to maintain the lateral relationship of the preform. The focusing process may be repeated to provide the desired diameter and/or length of the fiber, a focusing fluid and the narrowed structure can be further narrowed by repeated exposure to focusing fluid.
In another embodiment hollow fibers are produced. The hollow fibers are extruded from a source comprised of two concentrically positioned tubes. The center tube extrudes a gas such as air or a highly pure inert gas and the surrounding concentric tube extrudes molten silicate glass. The extruded silicate glass forms a hollow tube and is focused to a jet by the surrounding flow of gas in a pressure chamber. Multiple hollow fibers may be extruded simultaneously and joined together before solidifying, e.g. to form a photonic band gap structure.
An advantage of the invention is that the focusing pressure from the surrounding focusing fluid provides pressure distribution on the viscous liquid extruded or the preform and the pressure distribution can be calculated mathematically to show that it suppresses instability before any fiber drawing viscosity thereby indicating a theoretically unlimited increase in productivity.
Another advantage of the invention is that shear stress on the fiber produced from the extruded viscous material can be reduced to a minimum thereby allowing the controlled production of complex fiber structures including hollow fibers which can be combined to produce any desired configuration of photonic bandgap structures.
Yet another advantage of the invention is that gas temperature distribution along the nozzle is very rapidly transferred to the drawn fiber material thereby providing a means for a simple and accelerated control of the fiber temperature profile and offering a robust and simple manner of controlling the fiber quenching process and enhancement of fiber quality.
An advantage of the invention is that the optical fibers formed are uniform in size and shape along this length and are created with a relatively small amount of energy.
Another advantage of the invention is that it allows multiple fiber extrusions to take place simultaneously, thus allowing the fibers to be extruded as a bundle.
Yet another advantage of the invention is that the fibers can be extruded as a coated fiber using concentric needles in the devices of the invention.
Yet another advantage of the invention is that optical fibers can be created without contamination, resulting in optical fibers without localized weak points in the fiber caused by such contamination.
Yet another advantage of the invention is that fiber forming and stability using the production methods of the invention can be enhanced using an appropriate external pressure distribution.
Yet another advantage of the invention is that fiber stress can be dramatically reduced upon extrusion of the devices of the invention, as glass to solid contact is avoided due to the extrusion of the glass surrounded by the focusing gas or liquid.
Yet another advantage of the invention is that the device of the invention will have minimal contamination and/or clogging from the extrusion of the fiber, as the exit orifice never touches the fluid or perform.
Yet another advantage is that fiber quality is enhances by rapid fiber quenching owing to the coflowing gas expansion.
Yet another advantage of the invention is that complex fiber concentric structures can be formed by the dramatic reduction of radial viscous stresses of the present methods as compared to classic techniques.
Yet another advantage of the present invention is that when preforms are used they are not subject to fluctuations in shape based on the focusing procedure, and thus the drawn fibers are not subject to severe temperature fluctuations as with the use of plasma fireballs.
Yet another advantage is that the extrusion methods can be designed to create fibers with discrete functional elements based on the orientation of extrusion. This allows the production of specialized fiber structures, such as photonic bandgap structures, in conventional length fibers.
Yet another advantage is that the present methods can be used with preforms having very distinct structural elements, since the integrity of the relationship of the structural elements is maintained in the focusing procedure.
Yet another advantage is that the present methods can be used with larger diameter preforms.
These and other aspects, objects, features and advantages will become apparent to those skilled in the art upon reading this disclosure in combination with the figures provided.